Our portfolio is 768 residential units spread between 37 locations. We have 3 full time office staff, 3 part time and 2 interns. (There's also a maint crew of about 15-20 depending on the season.) For what it's worth, the other local PM cos tell us we're insane to be so lightly staffed.
I've always used the rule of 1 full time employee per 100 units. So I don't think it's unreasonable for you to ask for a part time assistant, especially given the range of your portfolio.